Can you use Cuda from GPUs of different generations?
Cycles will be able to use them both simultaneously with the CUDA backend, each will render separate tiles. For several reasons though, the result will probably be slower than just using the 2060.

Since each card works on a different tile and there's no way for the other card to steal a tile once the first card claims it, you will eventually end up in a situation where all but one tile has finished rendering and the 660Ti is working on the last one while the 2060 sits idle. Because the performance difference between the cards is so great, this wasted time will likely be more than you'd save by having the 660Ti help in the first place.

Both cards need to have the scene copied into VRAM in its entirety. As a result, you're effectively limited to the VRAM of whichever card has less. Which is the 2GB 660Ti instead of the 6GB 2060

The 2060 has RTX hardware units, which Cycles can use via the OptiX backend to speed up ray intersections. However, the 660Ti is too old to use the OptiX backend. And the 660Ti is so slow in general, it is likely to contribute less than the RTX hardware on the 2060 could.


So while it's possible to render on both cards simultaneously, it is more likely to slow down your renders than just using the 2060 alone.